This application claims benefit of Japanese Patent Application No. 2001-116749 filed on Apr. 16, 2001, the contents of which are incorporated by the reference.
The present invention relates to array waveguide gratings used as light wavelength multiplexing/demultiplexing elements for optical communication, array waveguide grating modules, optical communication units and optical communication systems using the same array wavelength lattices. More specifically, the present invention concerns array waveguide gratings with improved light signal frequency characteristics, array waveguide modules, optical communication units and optical communication systems using the same array waveguide gratings.
With processes of usual time internet connection and communication data capacity increase, demands for large capacity data transfer are increasing. In the optical communication using light signals, it is very important for large capacity data transfer to improve the degree of wavelength multiplexing. In this respect, the role of array waveguide gratings as multiplexing/demultiplexing elements for multiplexing and demultiplexing light wavelengths is important, and the array waveguide gratings are thought to be one of key devices. The array waveguide grating has a passive structure, and also has a narrow light wavelength transmission width and a high extinction ratio. The array waveguide grating also has such features as that it can multiplex and demultiplex a number of light signals in correspondence to the number of waveguides.
Such array waveguide grating is desirably free from sudden changes of its output level or loss level with variations of the laser output light signal frequency from the center optical frequency of each optical waveguide. Also, where multiple stages of array waveguide gratings are connected, the modulation components of the light signal are cut off outside a bandwidth, in which the individual array waveguide gratings commonly transmit the light signal. Thus, it is important from the standpoint of improving the light signal transmission efficiency as well to realize a transmission characteristic with a flat peak level with respect to optical frequency.
FIG. 33 shows an example of such array waveguide grating. The illustrated array waveguide grating 10 has a substrate 11, on which one or more first channel waveguides (i.e., input channel waveguides) 12, a plurality of second channel waveguides (i.e., output channel waveguides) 13, a channel waveguide array 14 with a plurality of component channel waveguides bent in a predetermined direction with different radii of curvature, a first sector-shape slab waveguide 15 connecting the first channel waveguides 12 and the channel waveguide array 14 to one another and a second sector-shape slab waveguide 16 connecting the channel waveguide array 14 and the second channel waveguides 13 to one another, are formed. Multiplexed light signals with wavelengths λ1 to λn, are incident from the first channel waveguides 12 on the first sector-shape slab waveguide 15, then proceed with their paths expanded therethough and are then incident on the channel waveguide array 14.
In the channel waveguide array 14, the component array waveguides have progressively increasing or reducing optical path lengths with a predetermined optical path length difference provided between adjacent ones of them. Thus, the light beams proceeding through the individual array waveguides reach the second sector-shape slab waveguide 16 with a predetermined phase difference provided between adjacent ones of them. Actually, wavelength dispersion takes place, and the in-phase plane is inclined in dependence on the wavelength. Consequently, the light beams are focused (i.e., converged) on the boundary surface between the second sector-shape slab waveguide 16 and the plurality of second channel waveguides 13 at positions different with wavelengths. The second channel waveguides 13 are disposed at positions corresponding to their respective wavelengths. Given wavelength components λ1 to λn thus can be taken out independently from the second channel waveguides 13.
FIG. 34 shows, to an enlarged scale, a boundary part between the first channel waveguides and the first sector-shape slab waveguide in the array waveguide grating shown in FIG. 33. The first channel waveguides 121 to 123, which are shown in a first boundary part 18 shown in FIG. 33 as well, have optical waveguides 211 to 213 having a rectangular shape with a width Wp and length L2 and terminating in the first sector-shape slab waveguide 15.
FIG. 35 shows a boundary part in the case of using parabolic or second degree function shape waveguides disclosed in Japanese Patent Laid-Open No. 9-297228. In this case, the first channel waveguides 121 to 123 shown in the first boundary part 18 have optical waveguides 221 to 223 having a second degree function shape with a length L2 and terminating with a width Wp in the sector-shape slab waveguide 15.
Insertion loss and transmission width are usually in a trade-off relation to each other. However, where rectangular optical waveguides 211 to 213 shown in FIG. 34 are used in lieu of the second degree function shape light waveguides 221 to 223 shown in FIG. 35, the transmission width can be improved without sacrifice in the insertion loss. It is thus a great merit to use the rectangular optical waveguides 211 to 213 shown in FIG. 34 for realizing a flat transmitted light frequency characteristic.
The above description has concerned with the shapes of the optical waveguides, which are disposed in the first boundary part 18 between the first channel waveguide 12 and the first sector-shape slab waveguide 15 shown in FIG. 33. Such optical waveguides 21 and 22 are provided for the purpose of providing for harmonic mode of input at their locality of contact with the slab waveguide to make the Gaussian waveform peak part as flat as possible.
In lieu of providing the above contrivance with respect to the optical waveguides 21 and 22, the same effects are obtainable by providing optical waveguides of the same shapes in the second boundary part 19 as the boundary between the second channel waveguides 13 and the second sector-shape slab waveguide 16. Here, for the sake of the simplicity of description, only the shapes of the optical waveguides in the first boundary part 18 will be considered.
Where the rectangular optical waveguides 211 to 213 as shown in FIG. 34 are used, the variable shape parameters are only the width Wp and the length L2 of the rectangular part. Therefore, if the width Wp and the length L2 can assume only values limited on the design, it is possible to change the characteristics in such ranges. In other words, in this case a problem is posed that the degree of freedom in fine adjustment and fine design for realizing various properties is very low. For example, the problem may concern the transmission width and the stroke in the trade-off relation to each other. These problems will be discussed in detail in the following.
FIG. 36 shows an ideal characteristic of wavelength multiplexed light signals. In the graph, the ordinate is taken for the transmitted light signal power level, and the abscissa is taken for the wavelength. The individual light signals 311, 312, 313 have a rectangular waveform and also have a maximum transmission width. Thus, signal components of other light signals are not mixed with the signal components of the intrinsic light signals. Where such ideal light signals 311 to 313 are multiplexed, by connecting multiple stages of array waveguide gratings or array waveguide grating modules the bandwidth of the individual light signals is not reduced. The center wavelength of the light signals 311 to 313 may be deviated, but the signal level is not varied. However, no light signal transmitted through such array waveguide grating has such ideal rectangular waveform.
FIG. 37 shows a summary of proposal of an array waveguide grating with a rectangular optical waveguide connected to a slab waveguide. In the Figure, parts like those in FIG. 33 are designated by like reference numerals and symbols. In this proposal, first channel waveguide 12 and first sector-shape slab waveguide 15 are connected to each other by a rectangular waveguide 33.
FIG. 38 shows a way of use of the array waveguide grating shown in FIG. 37 such that multiplexed light signal is spread as it is led from channel waveguide through rectangular optical waveguide and then taken out as light signals each separated for each wavelength. As a light signal 32 passes through a rectangular optical waveguide 33, it is changed to a harmonic mode light signal 34 and spread. The spread light signal is converged through a channel waveguide array 14 and at positions each peculiar for each wavelength. The converged light signal 37 is separated and taken out for each wavelength in such a form as to correspond to the position of a second channel waveguide 13.
FIG. 39 shows optical frequency characteristics of light signals taken out in the example shown in FIG. 38. As shown, individual light signals 37 are multiplexed with a high density, and skirt portions of adjacent light signals and also skirt portions of light signals at spaced-apart positions are complicatedly intrude in the wavelength ranges of intrinsic light signals.
FIG. 40 shows light signals of two adjacent channels. Light signals 331 and 332 shown by solid curves have a smaller transmission width T as shown by arrows than the case of light signals 341 and 342 shown by broken lines, but the influence of noise components due to cross-talk is less. However, the light signals 331 and 332 are sharper in waveform than the light signals 341 and 342, and therefore they are subject to greater loss in the case of deviation from the center wavelength. As shown, the optical frequency characteristic varies with the light signal waveform shape. For this reason, when building a communication system, it is necessary to determine the optical frequency characteristic of the array waveguide grating or the array waveguide grating module on the basis of a desire of giving preference to the transmission width or attaching importance to the cross-talk. For example, in the case of a trunk communication system it is possible that light signal is relayed at many places as it is transferred, and it is thought to attach importance to the cross-talk for minimizing the deterioration of signal. In the case of a terminal communication system, on the other hand, simpler circuit devices than those in the trunk system are used. In this circumstance, a certain extent of deviation from the center wavelength of each signal channel has to be allowed. In this case, importance thus may be attached to the transmission width.
Thus, as described before, with the rectangular optical waveguides 211 to 213 as shown in FIG. 34 the degree of freedom of changing the optical frequency characteristics in dependence on the circumstance with the array waveguide grating used therein is low. In this respect, the optical waveguides 221 to 223 having the second degree function shape as shown in FIG. 35 become attractive.
However, the wavelength multiplexing degree improvement demand is on a trend of being increased more and more. When the channel width of each light signal is correspondingly reduced, the gap width between the signal transmission widths of adjacent channel light signals are relatively reduced to strengthen the degree of inter-channel interference, thus resulting in relative cross-talk deterioration. In this situation, it is difficult to manufacture array waveguide gratings or array waveguide grating modules, which permit satisfactorily setting transmission width and cross-talk for meeting demands for various communication systems.